ABSTRACT
The North American practice for design of soil structures in geotechnical applications is based on a load and resistance factor design (LRFD) approach. Limit state design equations for failure or serviceability modes can be expressed as follows (AASHTO 2012; CSA 2006):
ϕ γR Qn Qi ni≥ ∑ (8.1)
where Rn is the nominal resistance for a particular limit state, φ is the resistance factor, Qni is the nominal load contribution, and γQi is the (corresponding) load factor. The load terms are due to permanent and live load contributions. For typical limit state design equations found in North American design codes, φ ≤ 1 and γQi ≥ 1. The expectation is that by satisfying limit state design equations expressed as Equation 8.1, design outcomes will have a probability of failure that is acceptable (i.e., small). For the case of a single load term, the limit state design equation can be expressed as
ϕ γR Qn Q n≥ (8.2)
The nominal values in the above equations are typically computed using closed-form equations that are deterministic and/or prescribed values (e.g., yield strength of a steel element). In LRFD design, the load terms are typically determined first and then the resistance value (Rn) is adjusted so that the limit state design equation is satisfied. LRFD calibration involves selecting values φ ≤ 1 and γQi ≥ 1 so that the probability of failure for multiple nominally identical structures does not exceed an accepted value.
